As universal signaling properties,NO(nitric oxide) involves not only in plant growth and development,but also in plant response to abiotic and biotic stress.Until now,its function of osmoregulation is unknown.The study,using NaCl as a stress factor and tobacco suspension cells as experimental material,we investigated the effects of NO on osmoregulation of tobacco cells under NaCl stress.The results showed that:(1)Endogenous NO of tobacco cells was induced by NaCl stress.During 100 mmol·L^-1 NaCl stress,production of NO was rapidly induced in cultured cells in vivo,and reached at the peak(with 2 times of the control) in 1 h after the stress treatment ,then the production was deduced rapidly until 3 h,from then on,the amount of NO was increased gradually and maintained at a high level until 48 h.(2)Exogenous NO significantly enhanced osmotic stress tolerance ability of tobacco cells.150 μmol·L^-1 NO donor sodium nitroprusside(SNP) significantly improved vitality and regrowth of the cells by 78.6% and 63.2%,respectively,lowered cell death by 48.5%,when compared to that of non SNP treatments tress cells.Further study indicated that SNP treatments led to relatively lower cell solute potential and higher water potential,which was benefit for maintaining cell pressure potential under the stress.(3)Exogenous NO significantly improved the synthesis and accumulation of proline in tobacco cells under NaCl stress.150 μmol·L^-1 SNP increased proline accumulation of the cells by 25.9% due to effecting the activity of proline metabolic key enzymes,including the activation of glutamate dehydrogenase(GDH),arginase and ornithine aminotransferase(OAT),and the inhibition of proline dehydrogenase(PDH).Meanwhile,SNP also had corresponding effects on the expression of GDH,OAT and PDH genes.Our results suggest that NO involved in osmotic adjustment of tobacco cells to enhance salty tolerance,in which NO regulated proline metabolism of the cells was an important mechanism.